Microprocessor circuits typically operate in two primary operating states, active mode and standby mode. In active mode, the circuits are executing processes or tasks, and are usually running at the specified operating frequency of the circuit. During active mode, most of the transistor gates are switching and relatively high operating frequencies are required. During standby or sleep mode, processing tasks are usually idle and relatively few gates are switching, thus requiring lower operating frequencies. The power consumed in transistor circuits is a function of both switching power and leakage power. Even when a circuit is in standby mode, potentially significant amounts of power can be consumed due to power leakage in the transistors. Therefore, while the power of a circuit operating in active mode is a combination of dynamic (switching) power and leakage power, with dynamic power being the dominant factor, in standby mode where few gates are switching, power consumption is largely a function of leakage power.
As device designs scale down to finer geometries (e.g., from 90 nm to 65 nm or smaller), circuits can run at lower voltages for given operating frequencies, thus reducing dynamic power consumption. However, leakage power increases exponentially as device geometries decrease. This is due to the fact that operating characteristics of the transistors change as the dimensions change, especially with regard to the effect of the threshold voltage VTH, which is the voltage at which a transistor switches state. In general, the decreasing dimensions of the transistors causes a scaling of the threshold voltage VTH relative to the supply voltage, which leads to an increase in leakage power. Because leakage power is the dominant factor in power consumption during standby mode, as circuit dimensions scale downward, the effect of leakage power becomes much more significant for this mode of operation. This effect can be an important issue with mobile or battery-operated devices that have limited power supply capacity, and which may spend significant amounts of time in standby or sleep mode, such as mobile phones, Personal Digital Assistants (PDAs), notebook computers, and similar devices.
A simple method of reducing power consumption due to leakage power that is presently used is to reduce the operating voltage of the circuit itself, but this obviously reduces the performance of the circuit and reverses the trend toward ever-increasing processor and circuit operating speeds. Another known method of reducing standby power consumption is to shift the threshold voltage levels of the transistors. In general, increasing the threshold voltage will decrease the leakage power, and thus reduce standby power consumption. However, increasing the threshold voltage also decreases the switching speed of the transistor, thus decreasing circuit performance when the circuit operates in active mode. Present methods of reducing leakage power are thus generally disadvantageous and do not adequately account for the operating mode of the circuits in order to dynamically and flexibly alter the threshold voltage of the transistors in accordance with the operating mode and other relevant parameters of the circuit.